The present invention is related to the field of data communications networks, and more particularly to networks employing ring architectures.
Ring architectures have been used in data communications networks to further certain operational goals such as improved availability. Ring networks have traditionally incorporated protection features that allow for continued operation even in the presence of some failures. Ring networks based on Synchronous Optical Network (SONET) standards, for example, have employed redundant, counter-rotating fiber rings along with mechanisms for routing working and protect traffic on the rings so as to enable the network to withstand some failures. More recently, a packet-based ring architecture referred to as Resilient Packet Ring (RPR) is emerging.
One desirable characteristic of RPR rings not generally found in prior rings, such as those based on SONET or the Fiber Distributed Data Interchange (FDDI) standard, is referred to as “spatial reuse”. This term refers to the ability to send unicast transmissions between two distinct points on the ring, rather than requiring each transmission to circulate around the entire ring. In traditional packet rings such as FDDI rings, packets are circulated around the entire ring and stripped from the ring when received back at the source node. In contrast, RPR rings employ destination stripping, so that unicast packets generally traverse only the portion of the ring extending between source and destination nodes. This enables the use of the remaining portion of the ring for other transmissions, resulting in generally higher network performance.
It has been known to use network devices referred to as “bridges” to link together multiple subnetworks to create a larger network. One area of traditional use for bridges has been in Ethernet local area networks. It has also been known to use bridges to link different rings, such as FDDI rings, together. Bridges generally operate at layer two of the seven-layer Open Systems Interconnection (OSI) model, which is the same layer at which many LAN and ring protocols are also defined. In particular, bridges are responsible for forwarding layer-2 data packets from one segment or subnetwork to one or more other segments or subnetworks. In some networks, such as Ethernet networks, bridges may include an address learning mechanism by which the bridge comes to know where at least some nodes are located. Packets destined for such known nodes can be forwarded using targeted “unicast” transmission. A bridge may also carry out this forwarding using “broadcast” transmission techniques, in which packets are transmitted on segments in a manner indicating that the packet should be examined by all nodes on each segment, because the location of the destination node is not known to the bridge. Transmission in FDDI rings, for example, always has a broadcast characteristic, because each packet is transmitted around the entire ring and examined at each node.
A potential problem with bridged rings is the need to transmit packets in a broadcast fashion when the destination node for a packet resides on a “remote” ring, i.e., a ring to which a bridge receiving the packet is not connected. Because broadcast transmissions must circulate entirely around a ring, they tend to reduce the degree to which spatial reuse can be exploited, and therefore tend to reduce the data-carrying capacity of the ring from what might otherwise be achieved. It would be desirable to provide for bridging of packet rings in a manner that helps preserve spatial reuse and thereby tends to maximize the data carrying capacity of the rings.